This invention relates to ink jet apparatus for ejecting droplets of ink, and more particularly, to ink jet apparatus having an ink supply system permitting more efficient head construction while providing improved fluidic compliance as seen in the manifold.
In the field of ink jet apparatus there is generally provided an ink supply system comprising a reservoir containing ink, a manifold for supplying ink to the inlet restrictors of an array of channels, and some form of flow path from the manifold to the relatively remote ink reservoir. In apparatus having a large number of channels, for example 32 channels, there is a problem of minimizing cross talk in the form of pressure disturbances and waves through the manifold. A cross talk type of disturbance is generally characterized by the development of a pressure impulse in the manifold due to the small volume liquid injection derived from the pulsing of a jet. In order to reduce such cross talk, one standard arrangement has been to design a manifold to present the inlet restrictor paths with as large a fluidic compliance as possible, the magnitude of the pressure wave being inversely proportional to such compliance. The value of compliance is a function of both the compressibility of the liquid volume and flexibility of the wall surrounding the liquid. The manifold compliance is also important to minimize the effects of external shock and vibration which, in certain situations, can lead to depriming of the apparatus. This approach has led to arrangements wherein a large compliance is achieved by forming a major portion of the manifold wall with a thin compliant diaphragm.
The construction of a manifold so as to maximize its compliance generally requires an expansion of the size of the manifold in order to achieve the necessary flexural compliance of the diaphragm. This results in an ink jet head which is larger than optimum, requiring the ink reservoir to be situated at a substantial distance from the manifold. This requirement has led to a variety of designs which generally contain a tortuous flow path from the reservoir to the manifold. In this instance, even though the reservoir itself represents a nearly infinite fluidic compliance, the impedance of the connecting path does not allow the manifold to take advantage of this compliance. Thus, the manifold design itself must essentially take on the entire job of minimizing the cross talk and the effects of external shock or disturbance, and generally fails to take advantage of the beneficial compliance characteristics of the reservoir. Arrangements for optimizing the compliance characteristics of the manifold have resulted in a tradeoff of a larger manifold configuration for a larger print head and relative displacement of the reservoir away from the inlet restrictors. The problem is thus solved at the expense of requiring a larger and bulkier print head, which is clearly disadvantageous.
Another problem which results from a large sized manifold is that of air bubble generation at the time of filling the apparatus with ink. The feeding of ink from a relatively small inlet to a relatively large manifold may result in excessive air bubble generation. Of course, if the manifold is large, then a further space penalty must be paid if the inlet tube is made large so as to reduce the air bubble problem.